Different syntaxes for writing regular expressions have existed since the 1980s, one being the POSIX standard and another, widely used, being the Perl syntax. They came into common use with Unix text-processing utilities. The concept of regular expressions began in the 1950s, when the American mathematician Stephen Cole Kleene formalized the concept of a regular language. Regular expression techniques are developed in theoretical computer science and formal language theory.
Usually such patterns are used by string-searching algorithms for "find" or "find and replace" operations on strings, or for input validation. ) (?= ) At least two spaces are matched, but only if they occur directly after a period (.) and before an uppercase letter.Ī regular expression (shortened as regex or regexp sometimes referred to as rational expression ) is a sequence of characters that specifies a search pattern in text.
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